Method of absorbing and condensing digestion gases and vapors



July 11 1939- A. D. MERRILL E-r AL 2,165,644

METHOD oF ABsoRBING AND coNDENsING DIGEsTIoN GASES AND vAPoRs Filed June26, 1935 2 Sheets-Sheet l Een l July 11;'1939- A. D. MERRILL E1- AL72,165,544

METHOD OF ABSORBING AND CONDNSING DIGESTION lGASES ND VAPORS Filed June26, 1935 2 Sheets-Sheet 2 Wwf/jf Patented July 11, 1939 UNITED STATESMETHOD F ABSORBING AND CONDENSING DIGES-TION GASES AND VAPORS Albert D.Merrill and Thomas L. Dunbar, Watertown, N. Y., assignors to ChemipulpProcess Inc., Watertown, N. Y., a corporation of New York ApplicationJune 26, 1935, Serial No 28,580

17 claims.

This invention relates to improvements in the digestion of fibrousmaterial, and more particularly to an improved method of and apparatusfor absorbing and condensing gases and vapors evolved during thedigestion treatment.

It has been proposed heretofore to utilize a flowing stream of acid towithdraw and condense gases generated or evolved in a pulp digester aswell as from other units of a digestion system. A

l0 typical example of such use is the employment of an eductor forwithdrawing gases and vapors from the vapor space of a digester. In suchprior use relatively cold acid Was pumped, at relatively high velocity,through the nozzle of an eductor and this stream utilized to induce aflow of gas from the digester. Such eductors usually comprised a singlegas inlet and a single nozzle with a valve means associated with thenozzle to control the flow of liquor therethrough.

To date but little attention has been paid to the proper design of sucheductors. Ifhese units, however, are very important in respect of theefficient operation of a pulp digestion system since they perform anumber of important functions.

They serve in the rst instance as a pressure con- Ytrol means to adjustand/or maintain a desirable loW pressure in the digester becauseof the Afact that this pressure is due substantially completely to condensiblegases (SO2) and vapors (H20) densation therefore largely controlsthepressure in the digester. Secondly such eductors serve as a combinedheat exchanger and mixer inasmuch as they provide a zone in which coldacid is caused to condense and absorb hotter gases and vapors, thusuniformly increasing the temperature and concentration of the acid.

As a result of considerable experimentation We have found that ifneductor is properly designed' 40 its eiliciency can be very greatlyincreased. The novel construction and operatinlof the improved eductor,tobe described, is based upon a recognition of factors, obtaining inthis art, which heretofore were either unappreciated or weredisregarded.

' An object of the invention therefore is to providean improved methodof gontrolling pressure in a pulp digester.

Another object is to provide a novel method of withdrawing andcondensing condensible gases' and vapors evolved .in a pulp unit.

A Afurther object is to condense condensible gases and vapors Yin a.pulp digestion unit by utilizing a plurality of streams of absorbingfluid.

Another object is to provide an eductor of im- The quantity or degreegof.such con-l (ol. 92-7) t proved design by reason of which optimuminduced flow of gases and vapors is secured.

Yet another object is to provide a novel method of controlling thequantity of gas flowing through anieductor.

A still further object is to devise means for in' suring optimum streamline flow of gases through an eductor.

An additional object is to devise an eductor in which a plurality ofzones are provided for the v 10 direct contact of liquid and gasesand/or vapors. With these and other equally important'and relatedobjects in view the invention comprehends the concept of providing aneductor of special design in which the size of the liquor inlet 15 pipeis specially correlated with respect to the position of the gas inlet soas to achieve improved results and in which improved adjustable controland optimum absorption are securedby utilizing a split or plural streamof the absorbing 2,0 liquid. By reason of the provisiony foradjustable`- control the eductor may be substantially standardized so asto fulfilll the requirements of plant units of varying capacity.

In order to enable a more ready comprehen# t2,5 I

sion of the underlying principles of the invention preferredillustrative embodiments are shown in the accompanying drawings, inwhich- Figure 1 is a diagrammatic illustration of a hot acid/ unitembodying the improved-operation andl 30 structure.

Fig. 2 is an enlarged longitudinal section of one modification of theimproved eductor. l Fig. 3 is a cross section taken on line 3-3 of Fig.2. f1 3 5 Fig. 4 is a central longitudinal section of another type ofeductor. v

Fig. 5 is a detail of the eductor shown in Fig. 4. Fig. 6 is adiagrammatic illustration of the type of flow of the fluids through theeductor. .540

As shown in the drawings and as is now known in the art, the majorelements of the apparatus include the acid storage tank I, hot acidaccumulator 2 and one or more digesters 3. Cold acid may be pumped fromthe acid storage tank by 45 means of pump 4 and forced through line 5and check valve 5 to the eductor l0, through main eductor branch-6,controlled by valve 6 and optionally through branch 'l controlled byvalve 1' in a manner more particularly to be described. 50 Cold acidfrom branch 6 passes at relatively high velocity through nozzle 8 andinduces a flow of` gasand/or vapor through the relief header 9. The coldacid .condenses and absorbs the hot relief gases and vapors and iscorrespondingly 55 and proportionately preheated. The combined uidstream then passes downwardly through drop leg II to second eductor I2.Here the fluids are contacted with a stream of acid recycled to and fromthe accumulator whereby further condensation and absorption is insuredand the pressure in line II correspondingly reduced. Such cooling andabsorbing stream is withdrawn from the accumulator through line I3, pumpI4 andV nozzle I5, thence through valve I6 back to the accumulator. Theflow of liquid through the recycle line may be controlled by properadjustment of valve I5'. As will be appreciated, since eductors I and I2are in series, maximum condensation and absorption, and hence maximumreduction in pressure in header 9, is secured, while' at the same timethe make-up acid entering through line is preheated.

In the operation of this type of plant, as is now known to those skilledin the art, the digester is first filled with chips by removing themanhole cover 20 and feeding the chips in by gravity or by means of achip packing device. After the digester has been filled with chips thevalve 2l' in the top relief line 2l is opened, valve 22' in side reliefline 22 and valve 23 in high pressure header 9 are closed. Valve 24 inthe low pressure relief branch 26 is closed and air vent valve 25 isopened. Hot acid is; then withdrawn from the accumulator 2 through theline 30 and forced by the digester-'filling pump 3l through header 32into the branch line 33. As the hot acid enters the digester the aircontained therein is correspondingly displaced and is discharged throughthe open valves 2| and 25 to the atmosphere.- lValves 2|' and 25' maybekept open until the digester is completely lled with liquor or until gasappears at the vent valve 25'. When such gas Vappears valve 25 may beclosed and valve 24 opened, valve 2l renaining open so as -to pass suchgas through the relief header 26 and thence through drop leg 2l to theacid storage tank. When the digester is completely filled valves 24' and2|. are closed.

Thereupon, that is to say when the digester is charged with chips and isfilled or substantially filled with liquor, operation of the-pump 3| maybe continued so as to force in addi-l tional hot acid and build up anydesired hydrostatic pressure within'the digester. In the usual `run pump3| is continued in operation until pressure of the order of from fiftyto ninety pounds per square inch is developed in the digester.Thereafter ow of acid from the accumulator to the digester may bediscontinued either by stopping the pump or closing a valve in thebranch 33.

Then, if desired, in order to insure a thorough penetration of the chipswith the hot acid and to equalize the temperature throughout thedigester, the digester recirculation circuit may be operated. This maybe done,'as will be noted, by opening valve 40', operating pump 4I, andthus withdrawing acid from the upper portion of the digester throughline 40 and discharging it into the lower portion of the digesterthrough line 42. This recycling ismaintained until the desiredconditions obtain, such conditions being a thorough penetration orsaturation of the chips with acid at a substantially uniform andequalized temperature throughout the digester. Suchprecirculation-soaking period may be maintainedA for a period of fromone-half to two hours.

At the end of the soaking period the chips within the digesterareuniformly penetrated with hot acid at a temperature of from 75 C. moreor less to about 109 C, Then, as more particularly explained incopending application of Thomas L. Dunbar, Serial No. 758,314, theliquid level within the digester is lowered` by drawing off reliefliquor through side relief line 22 and passing it by way of reliefheader 9, eductor I0 and drop leg II to the accumulator. Upon thelowering ofthe liquid level in the digester there is an abrupt drop inpressure from the existing pressure of 50 to 90 lbs. down to 30 lbs.more or less. This drop in pressure, as will be understood, permits theuse of low pressure steam for bringing the mass up to-cookingtemperatures. Upon lowering of the liquid level to the desired extentsteam is then admitted to the digester through steam lines (not shown)connected to the bottom cone of the digester.

This admission of steam soon builds up the pressure within the digesterand the temperature correspondingly increases at a rapid rate. When thepressure has been raised to the desired operating level it may bemaintained at this value by opening valves 2| and 23'. During thecooking operation the liquid level rises due to the condensation ofsteam. Some time after the initiation of the cook the top relief valve2l may be closed and the side relief line 22 opened so as to maintainthe proper liquid level. Thus in normal operation the side relief linemaybe opened at about the fourth or sixth hour of the cook. In themanner well known to those skilled in the art, by the propermanipulation of the valves 2| and 22 the desired pressure and liquidlevel may be maintained.

If desired, during the actual cooking or digestion process liquor may berecycled to andfrom the digester by operating the digester recyclepassed through line 26 and eductor fitting 28 through the drop leg 2 1into the acid storage tank. The eductor 28 may, if desired, be ofsubstantially the same type as eductor I0 or any other preferred design.

As explained in copending application Serial No. 758,314, the pressureconditions obtaining inthe system, and particularly in the accumulator,may be accurately controlled withina relatively narrow range. means ofthe vent line 50 connected at one end through the valve 50 to the-vaporspace of the accumulator and at the other through a fitting 5I to thedrop leg 2l. The valve 50' is preferably a pressure regulating valveadjusted to a predetermined setting so that when the pressure in theaccumulator exceeds this point gases This may be achieved byaccumulator. The valve 5I', like valve 50', is a pressure regulatingvalve. This may be set at any desired pressure differential of from theorder of 1 to 4 or more pounds. In thesecircumstances when the pressurein the relief line 9 exceeds by this amount the pressure in theaccumulator, the valve 54 automaticallyoperates and fluids are-by-passed around eductor i and vintroduced directly into theaccumulator. v

The digester and accumulator are provided, as is known by those skilledin the art, with pressure, temperature and liquid level' indicatingand/or recording devices.

This type of operation is greatly improved by utilizing the noveleductor. As shown in Figs. 1

and 2 the stream of cold acid forced through line 5 may be split into.streams, one passing through line 6 and the other passing throughline. 1. The acid entering through line 6 passes downwardly through thecentral pipe or channel 60 and is .ejected through the nozzle 8. Acidflowing from line 1 enters the eductor 'at a point approximatelyopposite the entrance of the relief fluids, that is to say in thearea ofthe outlet of header 9.

In the preferred operation acid is forced through lines 5 and 6 so thata relatively high pressure obtains at the nozzle 8. Operations have beenconducted in which the pressure at this `point is about 110 lbs. per sq.in. gauge. The size of the pipe 60 and the nozzle 8 is specially designed so that for normal operation of the plant the control valve 6,'is completely opened. The nozzle thus discharges the required amount ofliquor without utilizing the auxiliary stream passing through line 1; Iffor any reason the plant capacity is increased over the normal capacity,

the additional liquor requirements are satisfied by opening valve 1 andpassing the additional stream intothe eductor. In these circumstancesthe cold acid flowing through line 1 immediately contacts the gasesentering from header 9 and eil'ectsja'condensation and reduction ofpressure at this zone. The liquor flowing through the nozzle 8 atrelatively high velocity establishes a zone of' correspondingly reducedpressure which acts to induce 'a flow of fluids downwardly through theeductor. Y

In the preferred type of operation an auxiliary condensing stream forthe low pressure gases is also provided. As shown, a line 10, controlledby valve 10', is connected to the cold acid line 5. Such linecommunicates with the upper end of the drop leg 21 through a suitableeductor type of fitting 28. Cold acid flowing through the line 10 thuscontacts with low pressure gases flowing in through header 26 and cools,condenses and absorbs such gases. This 'condensation and absorptionreduces the pressure in line 26, causing a rapid ilow of fluidstherethrough.

Thecold acid pump 4 is so designed that it will maintain a pressure atthe vnozzle 8 of the order of 110 lbs. and will deliver 50% more liquorat that pressure than is actually required for a given operation.

As .shown in'Fig. 4, more rapid contact and,

consequent condensation of gases may be obtained by feeding the relieffluids into the eductor in-a plurality of streams. I'hus the tubular thesame zone as the liquid inlet 63, which latter is connected to theauxiliary-cooling acid line 1. As in the eductor shown in Fig. 2, themain conare intimately contacted with densing stream of acid enters theeductor through the central tube 60 andextends downwardly, below the gasinlet 6|-62, a predetermined dis tance, as will be more fully explained.In the op# eration of this type of eductor' streams of gases 5 enteringthe inlet 6i`62 respectively mutually impinge in the central area of theeducto'rand at v that point, of high turbulence, may be'contacted with acooling stream of acid admittedthrough branch line 1. The acid flowingdownwardly 1o through nozzle 8 establishes a reduced pressure at thiszone, ,which pressure sucks or aspirates fluids from the upper sectionof the eductor.l InA the area about nozzle 8, therefore, the relieffluids cold acid flowing at relatively high velocity.

In this type of eductor, therefore, there is pro*- vided two condensingand/or absorbing zones. The first or main zone in the area about nozzle.8' operates continuously and fullls the normal 4re- 20 quirements of agiven installation. A supplemental condensing zone is provided, asexplained, in

. the upper portion of theeductor and vcomes into play when cold acid isadmitted through the separate line 1. The specific mechanicalconstruction of -the eductor may of course be widely varied. Tofacilitate assemblage and servicing, that in the drawings is preferred.Thus a complete'unit may comprise a fitting 80 and the tube lill.Thisfiltting 30 may be a casting of a suitable corrosion `resistantmetal or alloy and ls provided with the terminal flanges 8| and 82.'Such fitting is formed with the gas inlet (or inlets) 83 which isprovided with the peripheral flange 84. The flanges are tapped in theusual manner to provide for attachment to a corresponding couplingflangeon the header 9. The eductor tting is also formed with the liquorrinlet pipe section 85, which is suitably formed to provide for tightattachment to the acid line 1.' o

- The tube preferably constitutesla separate member. This is formed atits `upper end with anr integral lateral flange, of greater diameterthan the bore of fitting '80, so as to seat upon the upper flange 8|.The tube is provided at suitable 5 points with spacer lugs 90 (see Fig.3). Such lugs are preferably of stream line form in cross section so asto diminish resistance to iiow.

The upper end of the drop leg Il may be formed with a flange I I' whichis tightly held between the bottom flange 82 of the'tting and thecoupling ring ill. It will be understood of course that/interposed'between the respectively cooperating flanges orabutting surfaces' aresuitably corrosion resistant packlngs -or glands.

` It is particularly to be observed that in a pre ferred structure theliquid-inlet 85 extends at an angle to the.central axis of the tube andto the axis of the header 9. In other words, liquor entering-through theline 1 and pipe -section I5 o impinges the entering stream of gastangentially, thus denect'ing the flow of gas downwardly and centrallyof the fitting and Vdrop leg.

As a result of considerable experimentation we have found that in orderto secure the best results in this type of operation, that is-to say inthe utilization of an eductor, the length 'of the pipe 60, or` itsequivalent. is very important and substantially critical. Specificallywe have found that in order'to obtain optimum results the length 70 ofthe tubetil, below the gas inlet, should be about etl ft. (seerzFig. 6).vWe have furtherfound, paradoxically, that variations in the diameter ofthe pipe II do not substantially affect this factor. In other words, wehave found as a mat-MA 45 burble angle.

50 cial relationship existing 5 that the velocity of the eillux streamfrom 'the nozzle 8 will vary with the diameter of this nozzle and as acorollary the pressure in the zone of the nozzle will correspondinglyinversely vary, that the requisite length of the pipe section would becommensurately affected. It is probable that within a certain degreethis is true but, as noted above, we have found that for usual diametersof the nozzle 8, the best results can only "be achieved by having thepipe section 60 of a ,length of the order of 4 ft. below the gas inlet.

A typical eductor which has successfully been employed comprises'onehaving an 8 inch bore, a central liquor pipe (60) of 3 inch diameter,and 1 l@ inch nozzle, and a gas inlet of 8 inch diameter. While,attempting no accurate rationale of this result, it would appear thatthe peculiar 'effectiveness of this particular length is due to stream'line flow, as diagrammatically. illustrated in Fig. 6. During operationgas enters the eductor tting from the line 9 at a pressure of the orderof 70 lbs.'or less.` As pointed out above, the fluid pressure of theentering liquid, at the nozzle 8, is of the order of 110 lbs. per sq.in. The gas entering the eductor fitting is deflected substantially atright angles and, due to its elasticity, is in a turbulent or burblingcondition for a considerable period of time, despite the rapid flow ofliquor` from the discharge nozzle 8. It would appear then that theeffectiveness of this particular pipe length is due to the fact that itinsures an egrtended length of travel, in which length the gases and /orvapors are extenuated from-a turbulent, swirling or burbling A.condition tonne of stream line or filament iiow. In other words we tbelieve the situation here is analogous to that obtaining toaerodynamics where, to insure the maximum lift, that is to say thegreatest continuity of airflow, the alrfoil must be maintained belowwhat is known in that art as the critical or At any rate we nd as amatter of actual fact that if the conditions outlined above are fulfilled,improved operations4 are secured.

We have found that in this art there is a spebetween the character and/orcondition ofthe condensable medium and the length of its travel underinduced iiow. -We have found further that in a typical pulp plantoptimum results in respect of pressure control and condensation may beachieved `by utilizing a relatively long length of pipe section 60, or,stated differently, by displacing the liquid eillux 'some considerabledistance from the gas inlet, such distance being suiiicient to insurevextenuation of the relatively elastic gaseous orvaporous medium from aswirling or turbulent motion into asubstantially stream line flow. Inthese circumstances liquid discharges from nozzle 8 into a substantiallyparallel encasing stream of gases and vapors, thus insuring optimuminduction effect, condensation and absorption.

i The novel eductor has been described particularly with respect to itsutilization in the high pressure relief line. It will be manifest,however,

that this improved structure may be employed at any other point in thepulp plant or in any other art or circumstances where the same resultsare l desired and comparable conditions obtain.-

Whilea preferred structure has been described,

it is`to be understood that this is vgiven for the purpose of explainingthe principle of operation of the device. The invention is considered toresidein the several broad concepts defined, such as the utilization ofa plurality of streams of condensing medium and the principle of optimumdisplacement of the nozzle from the gas inlet.

We claim:

l1. In a cellulosic pulp digesting process that method of recovering thechemical and thermal values of evolved gases which comprises admittingsuch gases'to an eductor and contacting the gas therein with a pluralityof streams of condensing liquid and utilizing the resultant liquid fordigesting cellulosic material.

2. In a cellulosic pulp digestion process that method of recovering thechemical and thermal values of evolved gases and vapors, which comprisesadmitting such gases to an eductor zone and contacting the gas thereinwith a plurality of streams of cellulosic pulp digestion liquor atdifferent sections of the zone.

3. That method of inducing a flow of gas from a digestion zone of acellulosic pulp digestion process in which gas is evolved whichcomprises admitting such gas to an eductor, contacting such gasimmediately with a stream of a relatively cool cellulosic pulpVdigestion liquor,fpassing the combined stream through a predeterminedtravel and then contactingA such combined stream with a separate streamof cellulosic pulp digestion liquor.

4- That method of inducing a flow of gas from a pulp digester of acellulosic pulp digesting process which comprises connecting the vaporspace of the digester through a conduit to an eductor, admitting astream of relatively cool cellulosic pulp digestion liquor to theeductor adjacent the area of admission of the gas, and admitting anotherstream of suchdigestion liquor to the eductor at a point displaced fromthe inlet of the gas,

5. In a cellulosic pulp digestion process that method of recovering thechemical and thermal Avalues of evolved gases and vapors which com- '7.vA method of reducing the pressure in a pulp.

digester and recovering the ohemicaland thermal units of evolved gasesand vapors which comprises lconnecting-the vapor space of the digesterthrough a conduit to an eductor, admitting a stream of cooling andabsorbing liquid to the eductor adjacent the inlet of the gases andvapors and in a direction tangential to the entering stream of gas,admitting a second stream of cooling liquid to the eductor at a pointdisplaced from the gas inlet and in a direction'substantially parallelto the iiow of gas at such point.

8. A method of withdrawing gases from a cellulosic pulping operatingplant unit in which such tor at a point suiciently displaced from thegas inlet that at such point the gases have assumed a non-turbulent,substantially stream line ow, and then utilizing the resulting liquorfor the digestion of cellulosic material.

9. A pulp digestion process comprising cooking a mass of fibrousmaterial in a digester with acid liquor, withdrawing gases and vaporsfrom the vapor space of the digester and passing such gases and vaporsto an eductor, contacting the gases in the eductor with plural streamsof relatively cool acid liquor and discharging fluids from the eductorto an accumulator.

l0. A method of reducing the pressure in a pulp digester in whichcellulosic material is pulped, and recuperating the thermal and chemicalvalues of evolved gases which comprises, admitting gases evolved in thedigester to an eductor in a plurality of streams and contacting such gasin the eductor with a stream of relatively cool cellulosic digestionliquor, and utilizing the resulting liquor for digestion of cellulosicmaterial.

11. A method of reducing the pressure in a pulp digester in 'whichcellulosic material is pulped, and recuperating the thermal and chemicalvalues of evolved gases which comprises, yadmitting gases evolved inthedigester to an eductor in a plurality of mutually impinging streams andcontacting the combined stream of gas in the eductor with a stream ofrelatively cool cellulosic digestion liquor, and utilizing the resultingliquor for digestion of cellulosic material.

12. In a cellulosic pulp digesting process that method of recoveringchemical and thermal values of evolved gases which comprises, admittingsuch gases in a plurality of streams to an educ# tor and contacting thegas in the eductor with a plurality of differential velocity streams ofrelatively cool cellulosic digestion liquor, and utilizing the resultingliquor for digestion of cellulosic material.

13. In a cellulosic pulp digesting process that method of recovering thechemical and Athermal values of evolved gases which comprises admittingsuch gases in a plurality of mutually impinging streams to anA eductor,admitting a stream of relatively cool cellulosic digestion liquor to theeductor in the zone of impingement and admittingj a second stream ofsimilar relatively cool di..v gestion liquor at a point displaced fromsaid zone of impingement and utilizing the resulting liquor for thedigestion of cellulosic material in the process.

14. A method of digesting pulp which comprises cooking a mass of fibrousmaterial with acid liquor in a digester, withdrawing gases and vaporsevolved in the digester andmassing them through a conduit to an eductor;forcing a, stream of relatively cool acid liquor from a con-v tainertoward' the eductor, splitting such stream of cooler acid and optionallyadmitting the split streams to the eductor at a plurality-of pointstherein; passing low pressure gasesfrom the digester through a secondconduit and contacting such gases with a split stream of acid derived .l

from the said first acid stream.

15. A pulp digestion process comprising cooking a massof brous materialin a digester with a digesting liquor comprised of a volatile component,withdrawing volatile components from the vapor space of the digester andpassing such volatilesto an eductor, contacting the volatiles in theeductor with plural streams of relatively cool digestion liquor anddischarging fluids from the eductor to an accumulator.

16. In a cellulosic pulp digesting process, that method of recoveringthe chemical and thermal valuesrof evolved gases which comprisesadmitting such gases in a plurality of mutually impinging streams to aneductor, preliminarily condensing the gases adjacent the point ofadmission by contacting said gases with a stream of relatively coolcellulosic digestion liquor and further condensing said gas by admittinga stream of relatively cool cellulosic digestion liquor to the eductorat a point displaced from said zone of impingement and utilizing theresulting liquor after such contact asa cellulosic digestion reagent.

17. In al cellulosic pulp digesting process, that method of recoveringthe chemical and thernial values of evolved gases which comprisesadmitting such gases to an eductor and connected drop leg, andcontacting and condensing the gases therein with a plurality of streamsof relatively cool cellulosic digestion liquor, said plural streamsrespectively contacting the gases at dierent points along the line of owof the gases in the eductor and then utilizing the condensing( liquid asa digestion reagent in the pulping process.

' ALBERT D. MERRILL. THOMAS L. DUNBAR.

